<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        *{
            padding: 0;
            margin: 0;
        }
        .Box{
            width: 100px;
            height: 30px;
            background-color: skyblue;
        }
        .transition-Box{
            width: 200px;
            /*height: 100px;*/
            height: 0;
            transition: all 0.5s ;
            background-color: purple;
        }
        /*.Box:hover+ .transition-Box{*/
        /*    height: 200px;*/
        /*}*/
        .father-Box:hover .transition-Box{
            height: 100px;
        }
        .father-Box{
            width: 100px;
            height: 30px;
        }

    </style>
</head>
<body>
<div class="father-Box">
<div class="Box"></div>
<div class="transition-Box"></div>
</div>
</body>
</html>